1. Field of the Invention
The present invention relates to a method of measuring an unevenness in exposure, and also to an exposure method using this measuring method, which can be suitably applied for an exposure process with the use of, for example, the so-called step-and-scan type exposure apparatus or the so-called slit-scan exposure type exposure apparatus wherein a reticle and a wafer scan a slit-like illumination area in synchronization with each other so as to sequentially expose a pattern on the reticle onto shot areas on the wafer.
2. Related Background Art
Heretofore, there has been used a projection exposure apparatus for a manufacture of a semiconductor element, a liquid crystal display element, a thin film magnetic head or the like with the use of a photolithographic process, in which a photomask or a reticle (which will be hereinbelow representatively denoted as "reticle") is illuminated by an illumination optical system so as to project and expose a pattern on the reticle onto a wafer (or a glass plate or the like) coated thereover with a photoresist through a projection optical system. As one of characteristics for evaluating such a projection exposure apparatus, the so-called image plane unevenness in exposure caused by unevenness in transmissivity of the illumination optical system or the projection optical system has been raised. Due to this unevenness in exposure, the width of lines of a pattern exposed onto the wafer have differed from a desired value, and accordingly, it has been required to confine this unevenness in exposure within a tolerable range during the exposure.
Accordingly, it has been required to precisely measure an unevenness in exposure. For example, in a conventional batch exposure type projection exposure apparatus (such as a stepper), an evenness in exposure has been measured as follows: a photo-electric conversion element (which will be hereinbelow denoted as "illumination sensor") such as a photomultiplier or a photodiode having a pin-hole like light receiving part, is set on a wafer stage on which a wafer is mounted. A photo-electric conversion element (which will be hereinbelow denoted as "integrator sensor") such as a photomultiplier or a photodiode for receiving a slight volume of light taken out from illumination light in an illumination optical system is provided in the illumination optical system. The reason why the later-mentioned photo-electric conversion element is called an integrator sensor, is that the integrated exposure value over a whole exposure field on the wafer can be known from an integrated value of an output signal from the photo-electric conversion element.
Further, conventionally, the illumination sensor has been moved to several points to be measured in the exposure field defined by the projection optical system, and accordingly, output signals from the illumination sensor have been integrated over the respective actual exposure time lengths. In this case, in order to eliminate affection by a variation in light emitting energy from a light source between points to be measured, the integrated value of the output signals from the integrator sensor has been also measured, and the integrated value of the output signals from the illumination sensor 18 has been divided by the integrated value of output signals from the integrator sensor for normalization. An unevenness in exposure within the exposure field has been measured from the thus obtained uneven integrated values of the output signals which have been normalized, for each of the measuring points. Further, these measuring points have been distributed in a desired area within the exposure field in order to measure an unevenness in an arbitrary area.
Recently, there has been a tendency to increase the chip size of a semiconductor element, and accordingly, large area exposure has been required for an exposure apparatus in order to expose a pattern having a larger area onto a wafer from a reticle. Further, there is a limitation on design and manufacture in order to increase the size of an exposure field defined by a projection optical system. In order to increase the area of a pattern to be exposed and in order to cope with the limitation to the size of the exposure field defined by the projection optical system, the so-called step-and-scan type and slit-scan exposure type projection exposure apparatuses (which will be hereinbelow representatively denoted as "scan exposure system") in which a reticle or a wafer scans an illumination area (which will be hereinbelow denoted as "slit-like illumination area") whose shape is rectangular, arcuate, hexagonal or the like so as to sequentially expose a pattern on the reticle onto shot areas on the wafer, have been developed.
Even in such a scan-exposure type projection exposure apparatus, it is required to confine unevenness in exposure on a wafer within an allowable range after exposure by the scan exposure system, the unevenness in exposure is one of important evaluation characteristics. In this regard, the scan exposure system also offers such a disadvantage in that an actual unevenness can not be measured after scan exposure only by measuring a static unevenness in exposure within an exposure field defined by a projection exposure system, as is in the case of a batch exposure system.